Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Quantos parceiros diferentes jogaram na final em 1973?
 ###Tabelas: CREATE TABLE world_championship_grand_prix_wins_by_engine_manufacturer (world_championship_grand_prix_wins_by_engine_manufacturer_id AUTO_INCREMENT PRIMARY KEY, Rank INT, Engine VARCHAR, Wins INT, First win VARCHAR, Latest win VARCHAR) CREATE TABLE ships_of_the_class (ships_of_the_class_id AUTO_INCREMENT PRIMARY KEY, Name VARCHAR, Pennant VARCHAR, Builder VARCHAR, Homeport VARCHAR, Commissioned VARCHAR, Status VARCHAR) CREATE TABLE release_details (release_details_id AUTO_INCREMENT PRIMARY KEY, Country VARCHAR, Date VARCHAR, Label VARCHAR, Format VARCHAR, Catalogue VARCHAR) CREATE TABLE mixed_doubles:_13_4_titles_9_runner-ups (mixed_doubles:_13_4_titles_9_runner-ups_id AUTO_INCREMENT PRIMARY KEY, Outcome VARCHAR, Year INT, Championship VARCHAR, Surface VARCHAR, Partner VARCHAR, Opponents in the final VARCHAR, Score in the final VARCHAR) CREATE TABLE radio (radio_id AUTO_INCREMENT PRIMARY KEY, Frequency VARCHAR, Call sign VARCHAR, Branding VARCHAR, Format VARCHAR, Owner VARCHAR, Language/Rebroadcast VARCHAR)  
SELECT COUNT Partner FROM mixed_doubles:_13_4_titles_9_runner-ups WHERE Year = 1973